The so-called tandem type color printer is known as an electrophotographic color printer formed by parallelly arranging photosensitive drums corresponding to yellow, magenta, cyan and black respectively.
The tandem type color printer includes a developing roller opposed to each photosensitive drum. An electrostatic latent image is formed on the surface of the photosensitive drum. When the electrostatic latent image is opposed to the developing roller following rotation of the photosensitive drum, the developing roller feeds a toner to the electrostatic latent image. Thus, a toner image is formed on the surface of the photosensitive drum. A sheet is transported by a belt to be successively opposed to the photosensitive drums respectively. Toner images of the corresponding colors are formed on the photosensitive drums respectively and transferred to the sheet in a superposed manner, to form a color image on the sheet.
A color printer having four photosensitive drums integrally mountable on/detachable from a main body casing is proposed as such a tandem type color printer.
The color printer according to this proposal includes drum subunits retaining the photosensitive drums respectively and a pair of side plates retaining these drum subunits from both sides in an axial direction of the photosensitive drums. Developer cartridges retaining developing rollers are detachably mounted between the side plates correspondingly to the drum subunits respectively. The drum subunits, the developer cartridges and the pair of side plates are detachably mounted on the main body casing as a drum unit.
Both end portions of the drum shafts of the photosensitive drums protrude outward from both side surfaces of the drum subunits. Four axial holes are formed on each side plate at regular intervals along a direction of the arrangement of the photosensitive drums. The end portions of the drum shafts are so inserted into the axial holes as to position the drum shafts (photosensitive drums).
The side plates must be formed in high working accuracy, in order to accurately position the drum shafts. However, the side plates are provided with light passing holes for passing detection light for detecting the quantities of the toners remaining in the developer cartridges respectively and flange portions for guiding mounting/detachment of the drum unit on/from the main body casing, in addition to the axial holes. Therefore, the working cost for the side plates is disadvantageously increased.
Further, the side plates are relatively increased in size due to the light passing holes etc. formed thereon. A hardly deformable high-priced material such as metal or resin having a low coefficient of linear expansion is employed as the material for the side plates in order to accurately position the drum shafts, hence the cost for the material is disadvantageously increased if the side plates are large-sized.